It's complicated

The kineticist is a lot of things. It is an incredibly clunky, unintuitive, and poorly-designed class, full of trap options, complexity for the sake of complexity, opaque rules, and poor layout (worse than Magic of Incarnum, for those who’re familiar with 3.5). Because of this, any discussion of what tier it falls into, and how strong it is, has to include a talk about floors and ceilings.

True Floor: tier 5, maybe tier 6

There’s some necessary background on the kineticist that needs to be discussed. The biggest thing to note about the kineticist is that burn does not work how it seems to be intended. The kineticist, as presented in the book, is a “blaster.” They are written as a class that shoots area-of-effect elemental damage at enemies, and does it “all day.” In the conceptual part of the class, kineticist burn is an overcharge mechanism, where you can burn yourself to go to greater heights of power.

Despite being presented like this, this is not the case in the mechanics. Burn is a limiting mechanism, to keep the kineticist from being able to continue to contribute well all through the day, and the way it works causes kineticists to simply not function if played as blasters.

Kineticist “at-will” blasting (1d6 per two levels) is not enough to meaningfully fight enemies with a challenge rating near the character’s level, and kineticist daily-limited blasting only deals slightly more damage, without the debuffs (or even ability to keep blasting through multiple encounters) that spellcasters get with area energy blasting.

Thus, the only particularly viable method of playing a kineticist is to take enough burn to fuel Elemental Overflow at the start of the day, then take no other burn outside of emergency situations. This precludes blasting, because blasting requires you to use a composite blast in every round to keep up.

Without the ability to blast, the kineticist has a single tool to use: kinetic blade, a powerful level 1 form infusion that allows them to make melee attacks (and full attacks) with a kinetic blast.

Non-Blasting Playstyle: Tier 4

This is the definition of “tier 4,” as laid out originally by JaronK:

Tier 4: Capable of doing one thing quite well, but often useless when encounters require other areas of expertise, or capable of doing many things to a reasonable degree of competence without truly shining. Rarely has any abilities that can outright handle an encounter unless that encounter plays directly to the class's main strength. DMs may sometimes need to work to make sure Tier 4s can contribute to an encounter, as their abilities may sometimes leave them useless. Won't outshine anyone except Tier 6s except in specific circumstances that play to their strengths. Cannot compete effectively with Tier 1s that are played well.

If the kineticist’s player is aware of the stronger ways to play the kineticist, the class’s floor and ceiling are, effectively, right next to each other. There are very few ways to optimize a kineticist's combat abilities; it’s really just the following:

• Take kinetic blade
• Cap burn to Elemental Overflow’s limit and then don’t take more
• Take Weapon Finesse
• Get Dex and Con-boosting items, and later on a kineticist’s diadem
• Add combat feats to taste

When played in this way, I do not think the kineticist falls into tier 5 or below. To start with, the kineticist can fight fairly well. The bonuses from Elemental Overflow are really good, granting the kineticist strong physical stats, which they will use to enable kinetic blade and kinetic whip full attacks. However, much like a barbarian, fighter, or rogue, the kineticist is fairly limited in scope despite ostensibly offering many types of options. In the end, the options they do get aren’t the worst in the world. Here is an example build I’ve assembled:

(It's a google sheets link; there's a second tab of level-by-level DPR calculations)

This is a simple human water kineticist, using default point buy and a kinetic blade focus. The build is mostly a melee striker/off-debuffer. It's not particularly focused or obscure with its effects; I merely started with kinetic blade and built around that, so I could do math for a “generic” character of sorts.

Something to note here is that this sample water kineticist does not have all of their feats or talents chosen. The reason for this is that at a certain point, I ran out of meaningful combat options to take. In an actual-play situation, this would mean the kineticist could branch out some more to get added versatility, since they’ve hit a point where their combat tools are “done” being optimized.

At very low levels, this kineticist has trouble against enemies who resist their elements, but can fight about as well as a bard, inquisitor, or similar character. At mid levels (4-8ish), they start to come into their own, gaining a small set of useful utility effects, and past that, they can reliably deal damage enough to 1.5-round equal-CR enemies. At high levels, they will continue to get some interesting (if not particularly splashy compared to 5th through 9th level spells) abilities, and their combat math will keep up fine (composite blasts being usable for burst damage, but not good as a primary option because of the cost).

The damage dealt by the linked build is basically element-agnostic; I chose water because it had particularly interesting utility. Physical blasts deal more damage with slightly less accuracy, and run into resistances less, but most kineticists can function alright with kinetic blade past the very early levels (where they will run around hitting things with a morningstar or a ranged blast).

In addition, at later levels, a kineticist has access to a very useful pseudopounce: ride the blast. With a +1 conductive ranged weapon of any sort, a kineticist can fire a shot, hit with that shot, deal additional kinetic blast damage, and teleport next to the enemy as part of that shot, to continue their full attack with kinetic blade. It requires some feat investment, but though it comes online at level 12, ride the blast is possibly the most efficient tactical teleport in the game. It also becomes a 480-foot spammable teleport in any round where the kineticist doesn’t feel like full attacking. As a unique trick, it’s pretty good.

Basic combat competence isn’t the only part of being tier 4, though. Some classes (like the barbarian) can get into the tier by smashing everything in their way, but the kineticist, even after all of this, doesn’t ever get the ability to reliably one-round enemies. However, they do get some interesting utility effects that help shore up this weakness.

Kineticist Utility

Kineticist has some alright enough combat and out of combat utility, including some things that many classes simply can’t replicate. Of course, this is very element-dependent; some elements get way more useful tricks than others, and some are particularly situational. Some of the standouts, though, are good enough that I think it helps justify inclusion in tier 4, when added onto the combat competence.

As a note, all of the listed abilities are at-will and do not require burn, but many of them (particularly "combat" options like Aether's animate objects) require concentration to work, and can spend 1 point of burn to make them not require it.

As another note, a kineticist isn't locked to one element. They can branch into a second, and later a third element's utility powers if they want, and by higher levels, with many feat slots open, that's likely what they will be doing.

Aether

Aether’s primary gimmick is telekinesis in ways that other classes can’t really replicate. They’re pretty good at showing up the arcane trickster, frankly. At 2nd level, they can pick up telekinetic finesse to gain the ability to do any sort of fine manipulation at range (no extra actions required, no limits, simply "you can perform any sort of fine manipulation you choose within close range"), and among their abilities include the following:

• Kinetic Barrier (1st): Creation of short-lived transparent walls (that you can shoot through).
• Telekinetic Haul (2nd): Use 100 lb/level telekinesis at-will (spending 1 point of burn to make it 1,000 lb/level and not require concentration).
• Telekinetic Invisibility (3rd): At-will invisibility (with lowered, but still high stealth bonuses).
• Self Telekinesis (3rd) and Greater Self Telekinesis (5th):Super jumps, with an upgrade that makes it an oddball fly speed.
• Force Barrier (5th): At-will defensive spheres (as wall of force but only around you).
• Aether Puppet (5th) At-will animate objects (requires concentration unless you take burn).

As far as utility goes, they're fairly good, though. Telekinesis and invisibilty are good Hammers for which everything can be a Nail.

Air

Air kineticists don't get as much as Aether kineticists utility-wise, but still get a bit. Their standouts, from what I can tell, are this list:

• Air Cushion (1st): Always-up feather fall. Can work as a prerequisite for flight.
• Air's Leap (1st): Outright doubling of all jump distance (vertical and horizontal), along with +level to Acrobatics to jumping, and the ability to take 1 burn to double that distance again. This is useful at low levels, and a prerequisite for flight.
• Aerial Evasion (3rd): You get evasion.
• Engulfing Winds (3rd): Wind wall at-will, as long as you concentrate (or spend 1 point of burn to not do so).
• Wings of Air (3rd): Always up fly spell; requires air's leap or air cushion. This comes four class levels earlier than the other flight talents.
• Wind Manipulator (6th): At-will control winds, requires concentration unless you spend 1 burn, but hey, make some tornado winds whenever you want.
• Weather Master (8th): At-will control weather, but only for tornadoes and hurricanes. If you branched into Fire, you can create hot weather of multiple sorts, and if you branched into Water, you can make all the others.

Overall, Air's niche is that it has better mobility and can easily beat some low-level pits/walls/etc challenges, and at higher levels, might have the most world-changing power, in that at-will control weather talent.

Earth

Earth is relatively underwhelming up until 5th level talents. It feels a lot like something you want to go secondary in, but it can still support a primary choice well enough.

• Kinetic Cover (1st): This makes a fragile 5x5x5-foot wall that neither side can see through.
• Earth Climb (2nd): A climb speed for stone and earthen surfaces isn't much to write home about, but it's a prerequisite to earth glide.
• Tremorsense (3rd): As a move action, you can send out a sonar ping through the ground. By taking 1 burn, you can have it work for multiple rounds, and ignore concealment and total concealment miss chances against anything you "see" with it.
• Shift Earth (4th) and Greater Shift Earth (7th): The former lets you play Minecraft; the latter is standard action move earth, able to massively reshape the countryside if you spam it.
• Greater Tremorsense (6th): This is commune with nature at-will, letting you map and learn about areas quite easily.
• Earth Glide (5th): The real winner. Freely enter and leave earth and stone (anything but metal, even worked stone) like an earth elemental does.
• Stone Sculptor (5th): Stone shape at-will.
• Seismic Master (9th): Earth is one of the few elements to have a real "capstone." This is earthquake at-will, and it's hilarious, even if it's likely not particularly strong in many combats.

Earth is the best as mid and high-level sequence breaking, shaping or walking through walls, mapping areas, and everything in-between. They also have some decent kinetic blast types.

Fire

Poor fire. Fire is the second-most-common energy type in Pathfinder, is commonly resisted or blocked outright, and doesn't have a lot of conceptually-interesting utility effects. It does have a handful, at least, but this, of all the elements, is the one I'd call the worst.

• Flame Jet (3rd) and Greater Flame Jet (5th): Super-jump and fly speed.
• Smoke Storm (3rd): Makes a 20-foot AoE of sickening smoke, but it's Fort negates, and requires an open flame to center it on.
• From the Ashes (9th): Fire gets very little, but at least it gets a capstone talent. This one is an immediate action that costs 2 burn to save you from death, reviving you and healing you 5 hit points per level.

Fire isn't great. Mono-fire kineticists are probably tier 5, if I'm going to be honest.

Void

Void, like fire, is kinda terrible. A mono-void kineticist won't actually have enough talents in their specialization to pick them.

• Gravity Control (3rd) and Greater Gravity Control (5th): They're clones of flame jet. Super jumps and upgrade for flight.
• Gravity Master (9th): Reverse gravity, but you can pick the direction.

Same verdict as fire; "don't go all-in on void" is a bit sillier though, since a character can't do so. They will have to get universal talents or branch into another element instead of doubling down. On the upside, though, their Kinetic Invocation (feat that expands elements) choices aren't terrible: Animate Dead (requires onyx gems) as a 3rd-level talent, Command Undead as a 2nd-level talent, and Mind Blank as an 8th-level talent.

Water

Unlike Fire, Water is pretty good. Cold is often resisted (most common energy type in the game), but they have okay enough physical blasts, and, importantly, an insanely strong early-game utility effect: Kinetic Invocation will unlock Silent Image as a 2nd-level utility talent that costs 0 burn. The rest is middle of the road as far as utility goes (both in and out of combat).

• Kinetic Cover (1st): Works like the Aether one. Transparent barriers.
• Slick (2nd): Grease at-will. Lasts 1 round unless you take burn, but potentially good.
• Veil of Mists (2nd): Disguise self at-will. You can make this one of your daily burn uses to make it last until you rest any time you use it that day, instead of several minutes.
• Cold Snap (3rd): This one is a bit weird; you use it at the start of the day, and then can turn an aura of cold air on and off. At later levels or in chilly environments, it will heavily penalize enemy Dexterity scores in melee, which is very good when combined with entangling infusion for further penalties.
• Water Manipulator (3rd): I'm listing this mostly because it's situational, but potentially useful for handling pools of water with macguffins at the bottom. It lets you raise and lower water, and keeps the water in place for as long as you concentrate.
• Shimmering Mirage (5th): This gives you 20% concealment for the whole day.
• Ice Path (6th): A bit late compared to the others, but it's a way into the skies for you. Always-up, self-only air walk.
• Tidal Wave (9th): Inconsistent terminology aside, this talent emulates the tsunami spell by costing 1 burn.

Water's primary niche is that it's the best low-level utility element. Silent image and disguise self at-will is extremely useful for many sorts of adventures, and it has some good combat tricks, too.

Wood

Wood is not a good element. Its tricks are very focused and situational, but it's not quite as bad as Void. As a note, they forgot to actually print its basic talent (which allows gardening at range).

• Warp Wood (3rd): Warp wood at-will.
• Greensight (4th): You can see through plant material. Useful for hunting and forest encounters.
• Plant Disguise (4th): You can disguise as a Medium or smaller plant.
• Shape Wood (4th): Wood shape at-will.
• Plant Puppet (5th): This is aether puppet, but it only lets you animate plants.
• Forest Siege (9th): Greater siege of trees at-will (requires concentration, can take 1 burn to make it last all day). This will create a bunch of siege engines for you, if you find yourself needing a mundane army at 18th level.
• Wood Soldiers (9th): This one is better, creating four advanced wood golems for you that get benefits based on your secondary elements.

Wood is actually better than I expected when I first went to look, but it's not as good either early or late as Air, Water, Earth, or Aether.

Anyway, back to the point

I think that, when you combine the fact that the kineticist functions well enough (but not necessarily excellently) in combat if you build it right (a statement that applies to some other accepted tier 4s and 3s), and the fact that each kineticist will have access to a fair pile of unique tricks and powers, some of which are quite strong (Water's silent images, Earth's sequence breaking, Aether's freeform telekinesis game, and the like), the kineticist can be called tier 4.

That is, as long as you don’t try to blast. The kineticist’s true floor is very low; it’s nonfunctional in a way similarly to the D&D 3.5 truenamer class if one tries to fulfill the concept laid out in the initial presentation. While this is a bit disappointing, the result of a well-built kineticist once that fact is known is, I think, strong and versatile enough to constitute not being in tier 5. It does not get enough things to do, and can’t fight or utility well enough to be called tier 3, but I think that the options I’ve outlined here are enough to consider it "capable of doing many things to a reasonable degree of competence without truly shining" for some element combinations, or "capable of doing one thing well, but often useless otherwise" for others.

A final note about archetypes

Kineticist archetypes are a bit of a mixed bag; very few of them alter the underlying burn mechanics, but some remove or change Elemental Overflow to no longer be as good. Thanks to the kineticist's combat math with kinetic blade relying so much on Elemental Overflow, this can be quite painful for them. Here's a list of the archetypes, what they change overall, and how they may affect the kineticist's tier:

• Blood Kineticist: Replaces the 1st-level infusion and thus delays kinetic blade slightly (which is fine overall) but is otherwise just a specialized Water-element kineticist. Functions the same way as normal kineticists for tiers.
• Dark Elementalist: Not only does this archetype lower the kineticist's combat abilities, it requires that they kill level-appropriate (CR = level or greater) challenges to slowly build their bonus to be equal to a normal kineticist's Elemental Overflow. Tier 5, maybe 6; they do not gain their combat power until they have already won multiple combats in any given day.
• Elemental Annihilator: This archetype drops all of the kineticist's utility talents to give them a specialized type of full attack that deals less damage than kinetic blade (locked to 1d8+Con modifier damage, rather than scaling like kinetic blade does). It will do about the same in combat as a normal kineticist (potentially slightly better), but losing their utility talents is incredibly painful. Tier 5, maybe 6, depending on how look at those tiers. It's possible that a well-built PC using NPC warrior levels would outfight an elemental annihilator's devastating infusion full attacks.
• Elemental Ascetic: Replaces Elemental Overflow with a monk's Flurry of Blows and a small, scaling damage boost. They fight overall worse than a chained monk, and unlike a normal kineticist, do not have a Constitution focus to handle the damage they're taking from burn to use their new class features. Tier 5, maybe 6, depending on how you look at those tiers.
• Elemental Purist: This archetype is an alternate way of doubling down on your first element. It functions about the same as a normal kineticist with respects to tiers.
• Kinetic Chirurgeon: They lose kinetic blade access entirely, and instead gain kinetic healer as a bonus talent. This wild talent is particularly weak as healing, thanks to its slow scaling and requiring you to take burn (or inflict burn) to use it. Tier 5-6, again depending on how you look at those tiers.
• Kinetic Knight: This archetype gains an improved kinetic blade as a bonus infusion, as well as some abilities that make it better at combat without losing any utility talents. Overall, it's a bit of a winner, though it does give up ranged versatility and ride the blast shenanigans. Still tier 4.
• Overwhelming Soul: This one trades away Elemental Overflow's stat increases for being Charisma-based and not taking burn. It's about the same as a normal kineticist for kinetic blade use, except that it has lower attack bonuses and better social skills.
• Psychokineticist: It's like a kineticist, except you can't use your abilities if you're buffed by your party bard, suffering from a fear effect, or otherwise being tagged with emotional abilities. It also does not boost physical stats with burn, and instead of negating your Con focus, it makes you instantly lose to most mental effects. This is a significantly worse kineticist in almost every way.

So that's the kineticist archetypes and how they alter this post's advice. Most of them are trap options, but a few, like kinetic knight, are okay enough.